This invention relates generally to biomedical analysis, and more specifically to methods and apparatus for conducting electrical current through a living subject to determine a property of the living subject.
At least some known methods use impedance cardiography to non-invasively determine cardiac output estimates. For example, impedance cardiography, sometimes referred to as thoracic bioimpedance or impedance plethysmography, may be used to measure the stroke volume of a heart. The stroke volume can then be multiplied by heart rate, for example obtained using an electrocardiogram (ECG), to obtain cardiac output. At least some known methods of measuring the stroke volume include modeling thoracic, or chest cavity, impedance ZT(t) as a constant impedance, Z0, and as a time-varying impedance, ΔZ(t). Changes in the impedance over time can be related to a change in fluidic volume, and ultimately stoke volume and cardiac output.
In at least some known methods, impedance is measured using an impedance waveform derived from two or more electrode assemblies placed at different locations on the living subject's body. The electrode assemblies include a stimulation terminal coupled to a current source and a measurement terminal coupled to a measurement device. AC electrical current supplied to the stimulation terminal flows from the stimulation terminal of a first electrode assembly through the living subject's body to the stimulation terminal of a second electrode assembly. Voltage at the measurement terminals of both electrode assemblies is then measured and used to obtain the thoracic impedance ZT(t). Known measurement and stimulation terminals are generally the same standard size on each electrode. For example, known electrode terminals generally accept identically sized electrical connectors from the current source or measurement device, respectively. However, because the electrode terminals are the same size, connectors may be inadvertently coupled to the wrong electrode terminal, such that the circuit is reversed. More specifically, the electrical connector coupled to the current source may be inadvertently coupled to the measurement terminal and the electrical connector coupled to the measurement device may be inadvertently coupled to the stimulation terminal. Coupling the electrical connectors to the wrong terminal may decrease an accuracy of the impedance measurement, which may decrease an accuracy of the determined cardiac output and/or may cause mismanagement of the living subject.